Iran may be one of the world’s most secular countries; some reports put mosque attendance in the Islamic Republic at just 2%, lower than Church of England attendance. When the odious Islamist regime falls at length, we probably will find that there are as few Muslims in Iran as there were Communists in Russia after the fall of the Berlin Wall. Like other religions rooted in traditional society, for example the nationalist-Catholic faith that Europeans abandoned after the two world wars, Islam cannot abide the onset of modernity. Some forms of religion can flourish in modernity; Islam is not one of them.
The variable that best predicts fertility across all Muslim countries is education: as soon as women become literate, they stop having children. That is a hallmark of a faith that melts away in the harsh light of modernity. David Goldman (aka Spengler)
For all of the noise about Islam rampant and Iranian religious aggression, the reality is that much of the Islamic world is in demographic decline.
Goldman thinks this may explain a good deal about the belligerence of the current Iranian regime. I suspect he is right. But it is not just Iran. Virtually every Muslim nation is experiencing a sharp decline in birth rates.
Or, at least they are in the more secular, less fundamentalist sections of the society. Because the other demographic truism – faith leads to babies – means that the more militant/less educated sectors of the society are still having five or six children per woman. Which, in turn, means that the battle of the cradle may be won, in the mid-term, by the groups most stridently anti-Western and anti-modern.
Demography can be ideological – in parts of the Muslim world (and in parts of the West) – the decline of the secular birthrate has distinct political implications.
(The fundamentalist settlers in Israel have a birthrate three times higher than more secular Israelis.)
Jay Currie 
Doug Saunders should have read Goldman’s book.
Islam was never a threat to the west simply because of their overall numbers. The threat exists because they have a greater number of fanatics willing to use whatever means required to spread their religion to all corners of the globe. And that isn’t going to change simply because the birth rate in Islamic countries goes down.